1. Field of the Invention
This invention relates to grain-oriented electrical steel sheet and material having very high magnetic flux density for use in the cores of transformers and the like in which {110} &lt;001&gt; Goss texture orientation is promoted to a high level, and a method of manufacturing same.
2. Description of the Prior Art
As a soft magnetic material, grain-oriented electrical steel sheet is used primarily for the core material of transformers and other electrical devices, and with respect to magnetic properties therefore has to have good excitation and core loss characteristics. Usually a B.sub.8 (magnetic flux density at a magnetic field strength of 800 A/m) value is used to express excitation characteristics numerically and core loss properties are expressed as a W.sub.17/50 (core loss per kilogram of material that has been magnetized to 1.7 tesla at 50 Hz) value.
In recent years there has been a sharp increase in societal demands for energy saving and resource conservation, which has brought increased demands for lower core loss values and improved excitation properties in grain-oriented electrical steel sheet, with the demand for lower core loss properties being particularly strong.
Core loss consists of hysteresis loss and eddy current loss. Hysteresis loss depends on such factors as crystal orientation of the steel sheet (in other words, magnetic flux density), purity and internal stress, while factors such as electrical resistance, sheet thickness, grain size, magnetic domain size and steel sheet coating tension contribute to the eddy current loss.
After a long history of careful consideration in terms of production technology the limit has been more or less reached with respect to purity and internal stresses and the like. The silicon content of steel sheet has been raised in an attempt to increase electrical resistance and reduce eddy current loss, but a limit has been reached inasmuch as raising the silicon content degrades workability with respect to manufacturing processes and products. A number of attempts have been made to reduce eddy current loss by decreasing the thickness of the steel sheet, but in addition to the inherent difficulty of achieving the secondary recrystallization needed to obtain a Goss orientation there are a number of other problems involved, with respect to the manufacture of transformers and the like, and as for the same core loss thicker sheet is industrially preferable to thinner sheet, there is also a limit to how much sheet thickness can be reduced.
JP-B-51-12451 and JP-B-53-28375 describe methods for improving the core loss characteristics that a tension coating imparts to steel sheet, but the tensioning effect of these depends on the product orientation, which is to say, the magnetic flux density, and as described in pages 2981 to 2984 of the Journal of Applied Physics, Vol. 41 No. 7 (June 1970), the higher the magnetic flux density B.sub.8 the greater the tensioning effect becomes. Thus, with commercial high magnetic flux density electrical steel sheet with a B.sub.8 of around 1.92 tesla, there is a limit to how much the core loss characteristics can be improved. Techniques for lowering core loss by artificially fining domain size have been described by JP-B-58-5968 and JP-B-58-26405, but in these methods the core loss reduction effect depends on the magnetic flux density and is limited with respect to the degree of magnetic flux density in current commercial products.
Among the quickest ways to reduce core loss is by fining of secondary recrystallization grains, which was proposed by one of the present inventors in JP-B-57-9419. However, the fact that it is difficult to obtain high magnetic flux density when the size of secondary recrystallization grains is fined limits the use of secondary recrystallization fining as a means of reducing core loss. As the remaining means of reducing core loss, in JP-B-58-50295 the present applicants proposed a method of raising the magnetic flux density B.sub.8 from the current level of around 1.92 tesla to a more ideal 2.03 tesla (the saturation magnetic flux density of 3% Si--Fe steel). For the first time this method stably provided a product with a stable magnetic flux density B.sub.8 that far exceeded 1.92 tesla. However, the fact that the method involves the application of a temperature gradient during secondary recrystallization, and that application of the method to mill coil form sizes is accompanied by a large loss of thermal energy as one end is heated as the other end is being cooled, are major problems with respect to commercial implementation.